1. Field of the Invention
The present invention relates to a multi-junction solar cell. In particular, it relates to a multi-junction solar cell comprising a plurality of photoelectric conversion devices so as to utilize light of wide wavelength.
In this context, the multi-junction solar cell signifies a solar cell in which a plurality of photoelectric conversion devices are stacked along a direction of film thickness thereof and electrically connected in series.
The multi-junction solar cell includes an intermediate layer, which is a transparent conductive film inserted between the photoelectric conversion devices stacked along the film thickness direction and made of a material typically used as a transparent electrode in solar cells and liquid crystal displays.
2. Description of Related Art
Recently, a single junction solar cell utilizing a crystalline silicon substrate has been commercialized and widely shared the market. However, it involves problems of expensiveness of the crystalline silicon substrate and short of a silicon material supply.
Accordingly, there have been conducted research and development of an amorphous silicon solar cell in which a photoelectric conversion device comprising a thin amorphous silicon film is formed on an inexpensive substrate made of glass or metal.
The amorphous silicon solar cell, however, shows low photoelectric conversion efficiency as compared with that of the crystalline silicon solar cell. Further, its photoelectric conversion properties are gradually degraded by long-term irradiation of light which generates dangling bonds within the photoelectric conversion device (a so-called Staebler-Wronski effect). A prospect to overcome such drawbacks have not yet been in sight.
Also conventionally known is a multi-junction solar cell comprising a plurality of photoelectric conversion devices stacked on a single substrate and connected in series, the photoelectric conversion devices being made of substances having different forbidden band widths.
For example, commonly known is a multi-junction solar cell comprising a combination of photoelectric conversion devices made of amorphous silicon and those made of crystalline silicon (see Japanese Unexamined Patent Publication No. Hei 1 (1989)-289173).
In the thus constructed multi-junction solar cell, the photoelectric conversion devices are stacked in such order that the forbidden band widths thereof become greater as getting closer to a light receiving side of the solar cell so that light energy of wide wavelength is sufficiently extracted.
That is, the photoelectric conversion device absorbs photons having energies greater than its forbidden band width (Eg). Photons having energies lower than Eg are transmitted through the photoelectric conversion device, which will be a loss.
Therefore, by stacking the photoelectric conversion devices in the above-mentioned order, an energy loss is reduced, improving utilization efficiency of light per unit area.
Further, the multi-junction solar cell shows various advantages, e.g., higher open circuit voltage is obtained as compared with the single junction solar cell, degradation of solar cell properties is inhibited and so on. Thus, the multi-junction solar cell has eagerly been researched and developed as an effective possibility of realizing high efficiency and cost reduction.
At present, solar power generation will likely be a competition against other power generation systems such as thermal power generation and nuclear power generation. Accordingly, it is essential to achieve higher efficiency and lower costs in order to put it into wider use. In order to meet such demands, the following points need to be achieved:
1) Increase of a short circuit current density of the photoelectric conversion device determining a short circuit current density of the multi-junction solar cell itself;
2) Increase of an amount of light entering the multi-junction solar cell; and
3) Increase of open circuit voltage.
As a multi-junction solar cell considering the above-mentioned points, commonly known is a multi-junction solar cell in which a transparent conductive film is provided between two photoelectric conversion devices stacked in such order that the forbidden band widths thereof become bigger from a light receiving side of the solar cell (see Japanese Patent Publication No. 2738557).
In the multi-junction solar cell, a thickness of the transparent conductive film is appropriately selected. Accordingly, reflectance of a surface of the transparent conductive film is maximized with respect to a wavelength of light absorbable in the photoelectric conversion device located at the light receiving side, and it is lowered with respect to a wavelength of light absorbable in the photoelectric conversion device located at a side opposite to the light receiving side.
This multi-junction solar cell seems to be effective in view of the above-mentioned point (1), but does not in view of the above point (2). This is because of the photoelectric conversion device whose surface is not uneven, whereas it is known as an effective for efficient introduction of light to the inside of the multi-junction solar cell to provide the photoelectric conversion device located on the light receiving side of the multi-junction solar cell with an uneven surface (hereinafter referred to as a surface of the multi-junction solar cell).
With the uneven surface of the multi-junction solar cell, reflection of the incident light is inhibited and a greater amount of light is introduced into the solar cell. Therefore, light is utilized with higher efficiency.
Further, in the multi-junction solar cell provided with the uneven surface, there is a large difference in refractive index between a transparent electrode formed on the light receiving surface of the solar cell (typically having a refractive index of about 1.5 to 2) and the photoelectric conversion device (typically having a refractory index of about 4). Accordingly, the incident light is greatly refracted and an optical path thereof is lengthened within the photoelectric conversion device, which increases an amount of the incident light absorbed in the photoelectric conversion device. Thus, the thickness of the photoelectric conversion device can be reduced. Hereinafter, this phenomenon is referred to as a light trapping effect.
With the photoelectric conversion device having the thus reduced thickness, formation time for the photoelectric conversion device in a manufacture line is reduced. Further, carriers generated by light irradiation move a reduced distance within the photoelectric conversion device, so that probability of the carriers caught by the dangling bonds existing in the photoelectric conversion device is reduced, particularly in the photoelectric conversion device made of amorphous silicon. Moreover, an internal electric field within the photoelectric conversion device is enhanced, which also helps the inhibition of the degradation of the solar cell properties due to light irradiation.
Thus, providing the multi-junction solar cell with the uneven surface is very useful for achieving high efficiency and low costs of the multi-junction solar cell.
As an example of the multi-junction solar cell provided with the light receiving uneven surface, Japanese Unexamined Patent Publication No. Hei 11 (1999)-214728 discloses a multi-junction solar cell having an uneven surface. In the multi-junction solar cell, the uneven surface is provided in accordance with an uneven surface of a lower photoelectric conversion device of crystalline silicon which is spontaneously generated during its formation and an uneven surface provided on a rear electrode.
However, according to this technique, the configuration of the uneven surface of the multi-junction solar cell is dependent on that of the lower photoelectric conversion device and that of the rear electrode.
Accordingly, conditions for forming the lower photoelectric conversion device and the rear electrode are limited, which causes difficulty in simultaneously optimizing both the film quality and the uneven surface configuration of the multi-junction solar cell. In particular, the film quality of the crystalline silicon greatly influences the photoelectric conversion efficiency of the solar cell. Therefore, the thus limited film formation conditions are not desirable from a viewpoint of improvement of the photoelectric conversion efficiency of the multi-junction solar cell.
Typically, the multi-junction solar cell is comprised of a plurality of stacked photoelectric conversion devices, in which various parameters construct complicated combinations. Accordingly, an improvement of one component often results in degradation of properties of other components. That is, in the present situation, a construction allowing optimization of every component has not yet been achieved.
Therefore, for realizing higher efficiency of the multi-junction solar cell, it is extremely important to create the construction which allows optimization of each of the components interacting with each other.
In view of these circumstances, the present invention has been achieved to provide a multi-junction solar cell of high photoelectric conversion efficiency by simultaneously optimizing the film quality and the uneven surface configuration of the multi-junction solar cell.
The present invention provides a multi-junction solar cell comprising: a support substrate having a first electrode layer, a plurality of photoelectric conversion devices and a second electrode layer stacked thereon, and an intermediate layer having an uneven surface being sandwiched between any two of the photoelectric conversion devices stacked adjacent each other.
The present invention further provides a multi-junction solar cell comprising a first electrode layer, a plurality of photoelectric conversion devices provided on the first electrode layer, an intermediate layer having an uneven surface being sandwiched between any two of the photoelectric conversion devices stacked adjacent each other and a second electrode layer provided on the top of the photoelectric conversion devices,
wherein at least one of the photoelectric conversion devices functions as a support substrate. That is, according to this constitution, one of the photoelectric conversion devices may be used as the support substrate.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.